Simplify opponent movecount reduction
This removes the reduction decrease that occured
when the previous ply had a movecount greater than
7.
Passed STC:
https://tests.stockfishchess.org/tests/view/65c3f6dac865510db0283ef6
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 11968 W: 3205 L: 2953 D: 5810
Ptnml(0-2): 38, 1310, 3064, 1506, 66
Passed LTC:
https://tests.stockfishchess.org/tests/view/65c42377c865510db0284217
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 35676 W: 9113 L: 8905 D: 17658
Ptnml(0-2): 22, 3893, 9802, 4097, 24
closes https://github.com/official-stockfish/Stockfish/pull/5040
Bench: 1148379